Reactance Relay: Construction, Torque Equation, Characteristics

reactance relay

In reactance relay, the operating torque is obtained by current while the restraining torque developed by a current-voltage directional relay.

The overcurrent element develops the positive torque and directional unit produces negative torque.

Hence, the reactance relay is overcurrent relay with directional restraint.

The directional element is designed that the maximum torque angle is 90˚.

Construction

The induction cup type construction can be used for the reactance relay. The schematic arrangement of reactance relay is shown in the figure below.

Construction of Reactance Relay
Construction of Reactance Relay

Here four pole structure is shown. It has operating coil, polarizing coil, and a restraining coil.

The current flows from the pole-1 to pole-3 through the iron stack. The voltage is used to energized the winding on pole-4.

The operating torque is generated by the interaction of fluxes due to winding carrying current coils. It means the operating torque is produced by the winding 1, 2, and 3.

The restraining torque is produced by the interaction of fluxes due to winding 1, 3, and 4.

Therefore, the operating torque is proportional to the square of current and the restraining torque is proportional to the product of V and I.

The desired maximum torque angle is derived from the RC circuit.

Torque Equation

The driving torque is proportional to the square of current and the restraining torque is proportional to the product of voltage V and current I.

Hence, the equation of net torque (neglecting the effect of spring);

    \[ T = K_1 I^2 - K_2 V I cos(\theta - \tau) \]

For the balance condition, torque is zero.

    \[ 0 = K_1 I^2 - K_2 V I cos(\theta - \tau) \]

    \[ K_1 I^2 = K_2 V I cos(\theta - \tau) \]

    \[ K_1 = K_2 \frac{V}{I} cos(\theta - \tau) \]

    \[ K_1 = K_2 Z cos(\theta - \tau) \]

The torque angle is adjusted 90˚ by adding capacitor.

    \[ \[ K_1 = K_2 Z cos(\theta - 90^\circ) \]

    \[ \[ K_1 = K_2 Z sin \theta \]

    \[ Z sin \theta = \frac{K_1}{K_2} \]

Consider an impedance triangle shown in the figure below.

impedance triangle
impedance triangle

    \[ Z sin \theta = X =Reactance \]

    \[ Z cos \theta = R = Resistance \]

    \[ X = \frac{K_1}{K_2} = constant \]

Therefore, it operates on the reactance. The constant X shows a straight line parallel to X-axis on R-X diagram.

The reactance observe by the relay should be smaller than the reactance for that the relay is designed.

Operating Characteristics of Reactance Relay

The resistance component of impedance has no effect on the operation of reactance relay.

The operation characteristics of the reactance relay is a straight line as shown in the figure below.

characteristics of reactance relay
characteristics of reactance relay

This straight line is a parallel to the X-axis (resistance axis) (R-axis).

All the impedance vectors have their tips lying on the straight line.

The relay responds only to the reactance component of impedance.

The relay will operate for all the impedances that lie below the operating characteristics.

The above part from the line is -ve torque region and below part of line is +ve torque region.

Disadvantages

The disadvantages of this relay are listed as below.

  • Reactance relay is a non-directional relay.
  • This relay will not be able to discriminate when used on transmission line, whether the fault has taken place in the section where the relay is located or it has taken place in the adjoining section.
  • It is not possible to use a directional relay of the type used with basic impedance relay because in that case the relay will operate even under normal condition if the system is operating at or near unity PF conditions.
  • This relay will have directional feature is called an admittance relay or mho relay.

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